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PEP 484: Do not require type checkers to treat a None default specially #689

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gvanrossum merged 2 commits into from
Jul 10, 2018
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PEP 484: Do not require type checkers to treat a None default specially #689

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Do not require type checkers to treat a None default specially
JelleZijlstra Jul 1, 2018
849f9b0
Be more explicit that we're moving away from implicit optional
JelleZijlstra Jul 10, 2018
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7 changes: 4 additions & 3 deletions pep-0484.txt
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Expand Up @@ -984,15 +984,16 @@ for example, the above is equivalent to::

def handle_employee(e: Optional[Employee]) -> None: ...

An optional type is also automatically assumed when the default value is
``None``, for example::
Type checkers may also automatically assume an optional type when the
default value is ``None``, for example::

def handle_employee(e: Employee = None): ...

This is equivalent to::
This may be treated as equivalent to::

def handle_employee(e: Optional[Employee] = None) -> None: ...

Type checkers may also require the optional type to be made explicit.
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@gvanrossum gvanrossum Jul 2, 2018

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Hm, maybe I was too wishy-washy. Pondering this again I think the language in PEP 484 should acknowledge that PEP 484 used to automatically infer Optional[] based on an = None default, and that type-checkers historically did it that way, but that we changed our mind and that in the future the PEP will mandate that this not be done. What type checkers currently do is then merely dependent on where the type checker is in its migration to the future behavior.

Does that make sense?

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@gvanrossum gvanrossum Jul 7, 2018

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@JelleZijlstra Any response to that?

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@JelleZijlstra JelleZijlstra Jul 8, 2018

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I forgot about this one, will get back to it soon.

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@JelleZijlstra JelleZijlstra Jul 10, 2018

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How is this new wording?


Support for singleton types in unions
-------------------------------------
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